How do I size a servo motor?

Work from the load back to the motor: required output speed and torque, then pick a gear ratio that lands the reflected inertia between 1:1 and 10:1 relative to the motor's rotor inertia. The Drivetrain Calculator shows the inertia match live as you edit.

What is an acceptable inertia ratio?

Between 1:1 and 10:1 reflected-load-to-motor inertia is the standard servo tuning range. Above 10:1 the motor starts to lose bandwidth and settling time rises. MotionConvert warns above 10:1 for this reason.

How is output torque calculated?

Output torque equals motor torque multiplied by the total gear ratio and by each stage's efficiency. Each gear stage's efficiency defaults to 100% (no loss) if left blank.

What's the difference between rotation and linear output?

Rotation output reports RPM at the last stage. Linear output converts that RPM into m/s (or another linear-speed unit) using a roll circumference, a roll diameter, or a spindle lead, depending on which output mode you pick.

Can I stack multiple gear stages?

Yes. Anonymous and free users can stack up to two stages; Pro users up to ten; Expert users have no limit. Each stage takes a ratio, a pair of teeth or diameters, or a saved component.

Drivetrain Builder

Configure motor, gear stages, and output.

Motor

Speed

Torque

Advanced

Rotor inertia

Fill rotor inertia + load inertia on the output to see reflected-inertia + match ratio.

Gear Stages

Stage 1

Ratio (1:N)

Advanced

Stage efficiency (%)

Anonymous users can add up to 2 stages. Sign in for more.

Output Stage

Output shown as RPM by default.

Roll Circumference

Roll Diameter

Distance per Rotation

Advanced

Load inertia

Target Values

optional

Fill these to see deviation from your calculated output.

Target Speed

Target Torque

Results

Total Ratio

—

Output Speed

Output Torque

Stage Breakdown

Stage	Ratio	Efficiency

Related Tools

Articles

Need more gear stages and saved configurations?

View Plans

Drivetrain Builder

Linear Output

Target Comparison

Inertia Matching

Stage Breakdown

Drivetrain Builder

Email PDF

Linear Output

Target Comparison

Inertia Matching

Stage Breakdown

Gear Finder

Unit Converter

Belt Length Calculator

Understanding Gear Ratios

What is a Drivetrain?